The ever increasing demand for wireless communication such as Cellular Mobile Telephone (CMT), Digital Cellular Network (DCN), Personal Communications Services (PCS), and the like, are giving rise to a corresponding need for new value-added services. These include services such as personal mobility, where a single personal telephone number may be used to direct a call to an individual user, regardless of where they are located. Other proposed services would also require that cellular telephones provide physical position location information. For example, the United States Federal Communications Commission (FCC) is presently considering a requirement for cellular service providers to have technology in place within the next five years to pinpoint the location of callers within 125 meters (about 400 feet).
Such services would not only provide personal security in the event of an emergency, that is they would provide the local authorities who respond to "911" calls with immediate information concerning the location of the disturbance, but would also enable other services such as vehicle theft tracking and recovery as well as equipment security and fraud detection.
The cellular telephone industry has resisted serious efforts in implementing position location services to date, however. This may be due to several considerations. First, there is a general fear that the cost of implementing such a system is high and that the required technology is unproven. For example, existing location systems require special purpose hardware and software to be installed in the mobile unit. With millions of mobile units already in use, any system which requires the implementation of new mobile hardware is likely to be an expensive proposition.
There may also be historic reasons why previous attempts to implement position location services in cellular systems have been less than successful. In dense urban environments, man-made structures such as buildings, over-head highways and the like impose multi-path distortion at cellular radio frequencies. This, in turn, complicates the position location process making it difficult to meet the requested accuracy specifications.